Various devices are known in the prior art for measuring the thickness of material. In the manufacture of a web or sheet of material such as paper, on line sensors are employed for measuring the thickness of the web. In many of these systems upper and lower heads are supported on an O-frame for movement together across the web to obtain a thickness profile. In one technique, one of the two heads carries a ferro magnetic disc and the other head carries winding on a magnetic core, the reluctance of which winding varies in accordance with the separation between the disc and the core.
In some of the systems of the prior art the sensor heads are in direct contact with the upper and lower surfaces of the web, the thickness of which is to be measured. In some instances the heads incorporate diaphragms or bellows affording a flexible contact with the web. Some heads of the prior art are of special geometric design and are covered with a smooth hardened material to minimize the effect of surface and strength irregularities of the sheet caused by moisture variations, dirt or holes which otherwise would cause tears or marks in the paper coating or web breaks.
Other expedients have been employed where the web carries a soft coating or has extreme surface irregularities. Such expedients include rolling contact between the head and the web or some provision for generating an air cushion between the head and the web. For rolling contact, small diameter rollers are employed. For fast production, the small rollers travel at such high rotational velocities that frequent replacement is necessary.
In sensors of the type using an air cushion, an attempt is made to maintain the distance between the head and the sheet material constant by maintaining a constant air column. It is, however, difficult to maintain air cushion distance precisely enough for the accuracy required in the measurement of thin webs.
In yet another system of the prior art the upper and lower heads are spaced from the upper and lower surfaces of the sheet or web. The respective distances between the upper surface of the sheet and the upper head and the under surface of the sheet and the lower head are measured by optical triangulation systems. A third measurement is made of the distance between the upper and lower heads. Using these three distance measurements, the thickness of the web is calculated. While such a system avoids the problems inherent in systems in which the heads contact the surfaces of the web, it is not as accurate as is desired, particularly in the case of thin webs.